Hey Heidi,
Apologies for the delay.
To be clear from the outset, we don't yet officially support Joomla 1.5 - jc_absolution was written for Joomla 1.0.X and also doesn't claim 1.5 support.
Nonetheless, you can get the template to work by switching on legacy support via:
Plugin Manager - set the Legacy System plugin to 'published'.
NIce guide here aswell -
http://www.mydigitallife.info/2007/11/29/how-to-enable-legacy-mode-in-joomla-15/
You will then be able to upload the template.
NOTE: As outlined above you may still experience issues with the template with Joomla 1.5.
We are due to upgrade our templates to 1.5 but as of yet we don't have a timeframe for that right now.
Thanks,
Ed